Technical Field
The present invention relates to a fluid treatment apparatus for treating a target fluid.
Description of the Related Art
Supercritical water oxidation apparatuses which decompose and detoxify persistent substances such as dioxins and PCB (polychlorinated biphenyl) and organic fluids such as human excrement, sewage, livestock excreta, and industrial effluent are known.
For example, a hydrothermal oxidation apparatus and a supercritical water oxidation apparatus each of which transforms a target fluid including an organic substance into a non-toxic substance, such as carbon dioxide, water, or an inorganic salt, by subjecting the target fluid to an oxidation reaction using supercritical water and an oxidant are known.
In such apparatuses, the reaction typically takes place under a pressure in a range of 25 to 50 MPa and a temperature in a range of 500° C. to 700° C.
Such apparatuses need to be resistant to high temperatures of about 450° C. to 700° C. and high pressures of about 25 to 50 MPa, which causes inevitable increase in their size and cost.
An apparatus which accelerates an oxidation reaction is also known. In this apparatus, a mixed fluid of high-temperature and high-pressure water, having a temperature equal to or greater than the critical temperature of water and a pressure lower than the critical pressure of water, with an oxidant is brought into contact with a catalyst in a reactor.
This type of apparatus using catalyst is capable of causing an oxidation of persistent organic substances even at relatively low temperatures of about 250° C. to 500° C.
In addition, this apparatus is capable of treating the target fluid under a milder condition (e.g., under a pressure in a range of 0.5 to 20 MPa and a temperature in a range of 100° C. to 500° C.) compared to the supercritical water oxidation apparatus. This contributes to downsizing and cost reduction of the apparatus.
One known method of arranging catalyst involves arranging a mesh-like container filled with granular catalysts 102 inside a cylindrical reactor 100 in such a manner that the granular catalysts 102 intersect with the direction of flow of a fluid, as illustrated in FIG. 1A. In this case, the fluid is allowed to migrate through interstices between the catalysts 102 while contacting the catalysts 102 in the axial (longitudinal) direction of the reactor 100.
In this case, however, flow resistance is large and treatment efficiency is low. Another known method of arranging catalyst involves arranging a honeycomb structural catalyst 106 inside the reactor 100, as illustrated in FIG. 1B. The honeycomb structural catalyst 106 is formed by bonding multiple tubes 104 extending in the direction of flow of a fluid. Each of the tubes 104 has catalyst layers on its inner and outer peripheral surfaces.
In this case, the fluid is allowed to migrate within each tube. Therefore, flow resistance is smaller than the former case that uses granular catalysts.
The honeycomb structural catalyst may be disposed downstream from the reactor relative to the direction of flow of the fluid so that the target fluid and the oxidant come into contact with each other while being sufficiently mixed with each other.
In many cases, a target fluid to be treated by such types of treatment apparatuses contains an inorganic substance (including an inorganic solid). The inorganic substance becomes solid and precipitates in the reactor.
Specific examples of the inorganic substance include alumina, silica, zirconia, phosphate, nitrate, sulfate, and the like.
Superheated water vapor and supercritical water have high dissolving power for organic substances but low dissolving power for inorganic substances.
In the reactor filled with the granular catalysts, the mixed fluid flows through intergranular spaces formed between the granular catalysts.
Among numerous granular catalysts filling the reactor, on those located on the most upstream side relative to the direction of flow of the fluid, inorganic solids will adhere or accumulate intensively.
The inorganic solids will eventually plug the intergranular spaces formed between the granular catalysts located on the most upstream side.
On the other hand, the honeycomb structural catalyst is less likely to be plugged with inorganic solids compared to granular catalysts, because multiple tubular spaces are secured, as illustrated in FIG. 2A.
However, inorganic solids 110 intensively adheres to the upstream end surface, relative to the direction of flow of the fluid, of the honeycomb structural catalyst because the upstream end surface is at a right angle to the direction of flow of the fluid.
The accumulated blocks of the inorganic substance 110 adsorbed to the upstream end surface of the catalyst will grow toward the center of the outlet of each tube, as illustrated in FIG. 2B.
When the outlets are not so large, the outlets will be plugged so early that the flow of the mixed fluid into the honeycomb structural catalyst will be interrupted.
In particular, when the oxidation treatment is performed under the condition where water exists as superheated water vapor, the density of the fluid becomes smaller compared to the case where water exists as subcritical fluid. In this case, it is difficult to wash the adhered inorganic solids away toward a downstream side.
Accordingly, the inorganic substances are likely to accumulate in the direction of gravitational force.